1. Field of the Invention
The present invention is generally related to pulse-width-modulated d.c.-to-d.c. converters, and more particularly to improvements in pulse-width-modulated converters employing zero-voltage-switching and improvements in pulse-width-modulating converts employing zero-current-switching.
2. Description of the Prior Art
Those skilled in the art have recognized the benefits and desirability of operating pulse-width-modulated (PWM) converters at high frequencies; high frequency operation allows a size and weight reduction of the converter for a given power rating. However, switching losses, component stresses, and noise due to parasitic oscillations are inherent problems with PWM technology, and these problems have limited, as a practical matter, the operating frequency of PWM converters.
Recently, new techniques have been proposed for high-frequency power conversion to reduce the switching losses in traditional PWM converters. Among them, the full-bridge (FB) zero-voltage switched (ZVS) resonant-transition PWM technique is deemed most desirable for many applications since it features the merits of both ZVS quasi-resonant (QR) and PWM techniques while avoiding their major drawbacks. Switching losses in the ZVS-PWM converters are significantly reduced without the penalty of a significant increase of conduction loss. Furthermore, the circuit operates with a constant frequency. However, due to a loss of duty cycle in the ZVS-PWM converters, the current and voltage stresses of the switches are significantly increased as compared to a PWM counterpart. In addition, parasitic ringing between the transformer leakage inductance (resonant inductance) and diode junction capacitance increases switching loss and switching noise.
FIG. 1 shows a prior art full-bridge ZVS-PWM converter comprised of a voltage source Vi, four switches Q1-Q4, four diodes D1-D4, inductor Lr, a resonant circuit comprised of Lf and Cf, and a load resistor R. The full-bridge zero-voltage-switching pulse-width-modulated converter can be viewed as two parallelled half-bridge zero-voltage-switching quasi-resonant-converters, where the phase-shift control is introduced. The full-bridge zero-voltage-switching pulse-width-modulated converter is a development of zero-voltage-switching quasi-resonant-converter technique. The zero-voltage-switching quasi-resonant-converter in the full-bridge zero-voltage-switching pulse-width-modulated are described in the following materials: "Zero-voltage switched quasi-resonant converters," by K. H. Liu and F. C. Lee, U.S. Pat. No. 4,785,387, Nov. 15, 1988; "Zero-voltage-switching technique in high-frequency off-line converters," by M. M. Jovanovic, W. A. Tabisz and F. C. Lee, IEEE Applied Power Electronics conference Proceeding, 1988; "Design considerations for high-voltage high-power full-bridge ZVS-PWM converter," by J. A. Sabate, V. Vlatkovic, R. Rideley, F. C. Lee and B. H. Cho, IEEE Applied Power and Electronics Conference Proceeding, 1990; "Full-bridge lossless switching converter," by R. L. Steigerwald, K. D. T. Ngo, U.S. Pat. No. 4,864,479, Sep. 5, 1989; and "Full-bridge power converter circuit," by L. J. Hitchcock, U.S. Pat. No. 4,860,189, Aug. 22, 1989.
Compared with a full-bridge (FB) pulse-width-modulated (PWM) converter, the effective duty ratio on the transformer secondary side, De, is decreased by: EQU D.sub.e =D/(1+(L.sub.r f.sub.s)/((N.sub.p /N.sub.s).sup.2 R))
where D is the primary duty ratio, R is the load resistance, and fs is the switching frequency. A larger resonant inductance corresponds to a smaller effective duty ratio, which in turn requires a smaller transformer turns ratio (N.sub.p /N.sub.s) to meet the line condition. Consequently, the primary current magnitude and the secondary voltage magnitude are increased, resulting in a reduction of the conversion efficiency. Another drawback to this converter is the severe parasitic ringing between the diode junction capacitances and the transformer leakage inductance. This ringing is more severe than that in a PWM converter since the transformer leakage inductance needed in the ZVS-PWM converter is considerably higher than that in a PWM converter. This ringing frequency is: ##EQU1## where C is the equivalent capacitance of the rectifier diodes and the transformer windings. The greater the transformer leakage inductance, the lower the ringing frequency. Lower ringing frequency leads to higher diode voltage stresses, higher snubber loss, and higher switching noise.
In ZVS-PWM or ZVS quasi-resonant converters, the active switches operate with ZVS, hence MOSFETs are usually preferred. For high-power applications, large voltage and current power devices such as BJTs, IGBTs or GTOs, must be employed. These devices, however, favor ZCS. Unfortunately, the high-current stresses of the active switches and the high-voltage stresses of the diodes in ZCS-QRCs, together with their variable-frequency operation, make the ZCS-QR technique less attractive for high power applications. On the other hand, as high power devices like IGBTs have relatively small output capacitances, the capacitive turn-on losses associated with PWM or ZCS operation, and parasitic ringing problems caused by the output capacitances of the IGBTs, become less severe as compared to MOSFETs. Furthermore, the operation frequency of IGBT circuits is considerably lower than that of MOSFET circuits.